During the last days of February
and first days of March 2001 Britain was affected by some cold and wintry
weather, including a good deal of snow in parts of Scotland and northern
England. Scotland recorded some very cold nights during this time too and
the minumum temperature of -20.6C recorded at Altnaharra (Highlands, northern
Scotland) on the morning of Saturday March 3rd was the coldest minimum
temperature registered in March in Britain since 1965 when, conicidentally,
on the morning of the 3rd Corwen (Denbighshire, northern Wales) recorded
-21.7C.

By way of comparison the coldest
March nights on record were the nights of March 12th 1958, when Grantown
on Spey (Aberdeenshire, north-eastern Scotland) registered a minimum temperature
of -22.2C, and March 14th 1958 when Logie Coldstone (also in Aberdeenshire)
dropped down to -22.8C. The coldest nights ever recorded in Britain were
11th February 1895, 10 January 1982 and 30th December 1995. On these 3
nights the mercury sank to -27.2C, on the first two occasions at Braemar
(close to Balmoral in Aberdeenshire) and in 1995 in Altnaharra.

So why did it get so cold then?
First of all it's no secret that clear nights are usually colder than cloudy
nights. Why? Because at night heat is radiated (ie given out) from the
ground and moves away from the earth. There's no incoming warmth from the
sun to replace this outgoing heat so the air near the ground gets cold.
Cloud cover will act like a blanket and prevent at least some of this heat
escaping, so that explains why clear nights are colder than cloudy nights.

Secondly, windy nights are
milder than still nights. Why? Well, on a still night the lack of movement
of air (which is all that wind is) means that air near the ground stays
in the same place all night and simply spends all night becoming colder
and colder. On a windy night the air near the ground is always moving so
can't cool off as fast. It's like bit like soup being heated at the side
of a pan but in reverse; if you are heating soup in a pan and don't stir
it the soup in the side of the pan will stay in the same and simply heat
up really quickly. If, on the other hand, you stir the soup every time
a bit of soup gets a bit hot the stirring will bring another bit of soup
to the side of the pan to get heated up. In other words stirring the soup
affects the rate at which soup at the side of a pan heats up - soup at
the side of a pan will heat up quicker if it isn't stirred. The reverse
happens on a still night - no stirring (or movement of air, ie wind), means
the air near the ground gets colder quicker than if the air was being moved
(ie it was windy).

Also, places that are in valleys
are colder on clear, still nights than places not in valley bottoms. Why?
Well, because cold air is denser than warm air it will sink down valley
sides into valley bottoms during cold, clear nights, making conditions
there even colder than they otherwise would be. This process is called
Katabatic Drainage.

OK, so that's why clear, still
nights will be colder than windy, cloudy nights and why places at the bottom
of valleys will be colder than places not in valley bottoms. Now, when
there's snow on the ground the air near the ground will be colder than
if the snow wasn't there, especially if the snow is fresh, deep and powdery.
Why? Because normally when the air near the ground is cooling off at night
some of this coldness will sink away into the ground. Snow, however, acts
a bit like a blanket over the ground and stop the coldness sinking into
the ground so the air near the ground winds up being colder than it would
be if snow wasn't there.

So, a clear, still night with
fresh, deep and powdery snow on the ground will produce the coldest nights,
and on such nights the coldest places will be in valley bottoms. Clear,
still nights occur when high pressure is overhead so the ideal conditions
for a really, really cold night are when heavy snow falls and then after
the snow has fallen skies clear and winds ease, or in other words when
a weather system produces snow over a region for a time and then high pressure
moves over and stays over the region in question for at least a night.

This
is exactly what happened over Scotland on the night of the 2nd/3rd March
2001. The chart to the left shows the weather situation on the night in
question. The front over the Baltic Sea and northern Germany brought some
snow to Scotland and northern England at the end of February and then as
the front moved away south-east an area of high pressure moved over Britain.
This area of high pressure brought clear skies and light winds to Scotland.
Bearing in mind that snow was already on the ground we can see that the
optimum conditions for a really, really cold night over Scotland were then
in place for the night of the 2nd/3rd March (as they had also been on the
nights before when temperatures dropped close to -15C in some parts of
Scotland) and in this respect mother nature didn't disappoint and delivered
Britain's coldest March night since 1965. You also won't be surprised to
learn that Altnaharra (and the other places mentioned in the first two
paragraphs!) are in valley bottoms!

This late February/early March
wintry weather wasn't just restricted to Scotland though. Snow affected
much of northern England too and in some parts of south-western England
saw snow lying on the ground for the first time since December 1997. The
Scilly Isles had their first lying snow since Febuary 1993 (Roger
Brugge's British Weather Diary). Further afield southern France and
northern Italy also had some snow during this time (50 cms snow fell in
some places, and in southern France it was some of the worst wintry weather
since the 1980's) whilst minimum temperatures in parts of Norway and Sweden
fell below -30C. Portugal and Spain were milder but extremely unsettled.
Here some places received some tremendous amounts of rain - including falls
of 50mm rain or in just less than a day in a few spots (Cordoba, southern
Spain for example).